An essential characteristic of a tomographic atom probe is the value of the mass resolution which can be obtained with this probe. The mass resolution of an atom probe is an essential quality which characterizes in particular:
the capability of the probe to unambiguously separate the various mass peaks of the different isotopes of the elements constituting the material to be analyzed. This ability to discriminate is commensurately more valuable when it relates to neighboring mass peaks with very different amplitudes.
the capability of the probe to increase the measurement accuracy of the composition of the material being analyzed, for example an alloy, by reducing the incorporation of detection noise at a mass peak corresponding to an element of the material being analyzed.
The mass resolution is expressed in the form of a ratio R=m/dm, where m represents the mass corresponding to the peak (i.e. to the element) in question, and where dm represents the width of the mass peak for an amplitude relating to this given peak. If m is equal to 28 and dm is equal to 0.28, for example, for h=0.5 (width at half height), then R=100 at half height.
Consequently, the problem posed for manufacturers of such probes, which constitute time-of-flight mass spectrometers, is to obtain the best possible spectral resolution. Obtaining a good mass resolution involves controlling the motion speed of the material particles from the sample from which they are detached, generally in ionic form, to the detector. In other words, the problem posed consists, as is known, in finding a way of making it possible for the atoms evaporated in the form of ions to be detached from the sample with the same initial potential energy and for them then to travel substantially at the same speed to the detector.
As is also known, the input of potential energy necessary for detaching a few atoms of an atomic layer of the sample is carried out by applying a voltage pulse of high value (high-voltage pulse), the duration of which is in practice of the order of one nanosecond, in the sample extraction zone (the tip).
This voltage and the curvature of the end of the sample are sufficient to create an electric field whose strength is enough to obtain the “field-effect evaporation” phenomenon, an effect which is known to the person skilled in the art. In practice, however, the production of a pulse quasi-instantaneously reaching the theoretical potential level required and constantly maintaining this level for a given time, before likewise ending almost instantaneously, constitutes a genuine real problem in the current state of the art. Consequently, pulses whose upper part has an approximately parabolic shape are generally produced, which leads to a degradation of the mass resolution when such pulses are used in a tomographic atom probe. Varying the amplitude of the pulse applied to the sample during the evaporation and then during the first few nanometers of the trajectory of the emitted ions, thus leads to the appearance of an energy (velocity) spectrum which is fairly broad instead of a single energy line. The spectral resolution of the atom probe is therefore contingent on the capacity to produce a square-wave pulse with a high amplitude and steep edges.
FIGS. 1 and 2 illustrate this dependency by simulated mass histograms for a tomographic atom probe or “Watap” according to the acronym for “wide angle tomographic atom probe”, the simulated probe having a given flight length of 0.11 m.
FIG. 1 illustrates a typical favorable case, taken by way of example, in which the evaporation pulses are square-wave pulses having a plateau of 200 ps and edges of 50 ps.
FIG. 2 in turn illustrates the less favorable case, in which the evaporation pulses are square-wave pulses having a plateau of 200 ps and edges of 1000 ps.
Both the presence of a peak 11, 21 having a certain width and the presence of a tail 12, 22, which is more extended but of lower level, can be seen in each of the figures. The peak here corresponds to the evaporation produced when the evaporation pulse has reached its maximum level (plateau of the pulse), while the tail in turn corresponds to the evaporation produced during the time intervals corresponding to the leading and trailing edges, and for which a loss of energy of the evaporated ions is observed.
Consequently, knowing that the relative amplitudes and durations of the peak on the one hand, and the tail (part of the pulse lying after the peak) on the other hand, characterize the resolution of the probe in question, and that the higher the amplitude of the peak and the smaller its width are, the greater the resolution of the probe is, it can be seen from FIG. 2 that extension of the rise and fall times of the pulse leads to a degradation of the resolution. In other words, the closer the evaporation pulse approximates a square-wave shape the higher the resolution of the probe can be.
In order to limit this problem, manufacturers are looking for ways of producing a constant amplitude plateau with steep edges, that is to say very short rise and fall times (typically less than 100 ps) for very short pulses (typically less than 500 ps). The known prior art proposes various approaches for producing such high-voltage pulses.
For instance, there are high-voltage pulse generation devices using a relay wetted with mercury. The repetition frequency of the pulses produced by such devices, of relatively old design, is however extremely low (of the order of one hundred hertz) in view of the desired characteristics, which makes the analysis of the sample relatively slow.
There are also semiconductor devices, of more recent design, which make it possible to produce short-duration pulses whose amplitude can be set in a wide voltage range, typically between a voltage of close to 0 V and a voltage of 4 kV. These devices moreover make it possible to obtain repetition frequencies extending up to a few tens of kilohertz.
However, these performances are obtained at the cost of a degradation of the rise and fall times, that is to say the steepness of the edges of the pulses produced in this way. In the current prior art, such devices therefore do not make it possible to produce high-voltage square-wave pulses of short duration, that is to say pulses having steep edges (typically less than 100 ps) and a short (typically less than 500 ps) maximum of constant level.
There are furthermore pulse generator systems capable of producing pulses having the desired temporal characteristics, but a lower amplitude which is fixed or difficult to adjust. These systems are therefore unsuitable or not very suitable for use in the scope of a tomographic atom probe which, by its nature, is intended for the analysis of different materials, each material requiring the production of pulses whose voltage value is proportional to the high voltage which polarizes the sample.
No known device of the prior art, therefore, is generally satisfactory in the context of producing atom probes with high mass resolution.